I will begin this with saying that I don't have any experience with 603's.
However, I saw a post not too long ago that mentioned a linkage problem creating a similar scenario. What had happened was the linkage on the side of the IP was shortened to the point that it was nearly in shut-off at the idle stop. Lengthened the rod and it idled beautifully.
If this helps, great.
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Sam
84 300SD 350K+ miles ( Blue Belle )
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